Certain types of foundry sand used in shaping moulds have several weight percent of resin added for the sake of its adhesive properties. Such foundry sand retains the shape of the mould after moulding on account of the caking force of the resin. However, the heat of the molten metal when it is poured into the mould causes the resin to carbonise and adhere to the surface of the foundry sand. The carbonised accretion adheres firmly to the sand and creates problems in that, among other things, regenerating the foundry sand in this state by adding fresh resin causes the resin content to increase, resulting in defects in the finished mould.
Consequently, if spent foundry sand is to be re-used, it is necessary to remove the accretion (carbonisation) adhering to it.
Conventional methods of regenerating whereby the accretion is removed from the foundry sand, which is then regenerated, include the following:
(1) Mechanical method whereby the accretion is removed from the foundry sand by applying friction though rubbing the sand together or on to rollers, or by beating the sand with an impeller or similar device and crushing the accretion by means of the impact. PA1 (2) Fluidised roasting furnace method whereby the accretion is removed from the surface of the foundry sand by applying flames from a burner and combusting it within a fluidised roasting furnace, the foundry sand being fluidised by blowing air from beneath. PA1 (3) Kiln baking method whereby the accretion is combusted and removed using a rotary or similar kiln to heat the sand with a burner from above or below while rotating it or otherwise causing it to move.
Nevertheless, while the mechanical method of regenerating has the advantage that the equipment used is relatively compact in comparison with that used for regenerating by combusting, it has proved difficult to remove the accretion totally even with long processing because it is burnt firmly on to the surface of the sand and a residue remains there after mechanical processing. For this reason it has not been possible to use the processed sand as if it were fresh.
In this respect, methods whereby the accretion is removed by combusting are preferable because the accretion, consisting as it does of carbides, is itself combustible and can be processed totally. However, were it just a matter of the accretion, its combustible components would be capable of self-combustion, but the amount of the accretion is only a small percentage of that of the foundry sand. As a result, the heat of combustion is almost all absorbed by the foundry sand before combusting the accretion: even if it ignites, it fails to combust continuously in a state of self-combustion.
It has therefore been normal to apply external heat as in the fluidised roasting furnace method and the kiln baking method, and to cause the foundry sand to rise or to move by means of a motor or other device.
Thus, in the case of the fluidised roasting furnace method, the foundry sand is roasted while a current of air causes it to flow within the furnace, as a result of which it is necessary for the flames of the burner to be applied constantly to the foundry sand. Moreover, a large amount of heat is required to heat the air which is injected into the furnace for the purpose of fluidising the sand. Most of the thermal energy supplied by the burner is used not for heating the foundry sand but for heating the air in order to fluidise the sand, with the resultant problem that thermal efficiency is low while the cost involved in regenerating is high and the apparatus cumbersome. Methods have been devised, for instance, whereby in order to provide a solution to the problem of thermal efficiency a heat-exchanger is fitted which preheats the air used for fluidisation (Japanese Patent SHO 64-2462), but they have not proved very effective.
The kiln baking method causes the accretion to combust forcibly by applying a burner while moving the sand. It requires a great deal of motive power in order to move all the sand, with the resultant problem that the apparatus is unwieldy and equipment costs are enormous. Moreover, fluidity of the grains of sand within the kiln is poor, so that while it is possible to combust the accretion in the vicinity of the burner and on the surface layer where the flames of the burner reach directly, the parts which do not come into contact with the flames become oxygen-deficient and the accretion simply undergoes thermal decomposition, the resultant compounds with a large number of carbon atoms adhering to the surface of the sand.